Many electronic components (e.g., infrared sensors) must be cooled to cryogenic temperatures to operate. Infrared sensors and associated electronics are often contained in a vacuum sealed housing commonly known as a Dewar assembly. The cryocooler assembly includes a “coldfinger” which has heat exchangers defining a cold end and an opposite warm end, and an expander removably positioned and extending between the warm and cold ends of the coldfinger. The expander includes a regenerator which operates to transfer heat from the cold end region to the warm end region of the expander while the cryocooler operates.
Standard Advanced Dewar Assembly II (SADA II) is a military standard that requires a coldfinger type cryocooler to have a specific geometry to allow “in-the field” integration into a Dewar assembly (e.g. at the Dewar/sensor manufacturer's facility). The expander must therefore be unitary to allow it to be “dropped-in” to the coldfinger by the Dewar/sensor manufacturer. Dewar/sensor manufacturers therefore often require cryocooler manufacturers to provide cryocoolers that are compliant with the SADA II standard.
One technology used in cryocoolers is known as a split Stirling cryocooler which comprises a rigid cylinder with an internal moving regenerator component that oscillates through a fixed quantity of working gas within the cylinder in response to pressure oscillations from an external compressor. As the regenerator component moves, gas is alternately compressed and expanded with the heat of compression being transferred from a “cold” heat exchanger located at the cold end of the expander to “hot” heat exchangers located at the warm end of the expander. When the cryocooler is installed in a Dewar assembly, the cold end is positioned closely adjacent or against the sensor to be cooled. Heat is removed from the cryocooler system at the “hot” heat exchangers in the warm end region of the pulse tube expander.
The Stirling regenerator and cold end heat exchanger are encased in a rigid cylinder to provide a unitary, self contained, cylindrical expander. The “warm end” of the expander attaches to the cooling head which includes the appropriate connections and tubing leading to a cryocooler compressor and buffer. The “cold end” of the expander extends outwardly therefrom and is inserted into the SADA II coldfinger which thereby completes the cryocooler assembly for shipment to the Dewar/sensor manufacturer. The coldfinger closes off the cryocooler unit to the ambient allowing the cryocooler unit to be charged with an inert gas which keeps the cryocooler clean during handling and shipment to the Dewar/sensor manufacturer.
It is common practice for the Dewar/sensor manufacturer to have already welded a SADA II coldfinger into their Dewar housing. Thus, upon receiving the cryocooler from the cryocooler manufacturer, the Dewar/sensor manufacturer must first remove the SADA II coldfinger from the cryocooler unit as shipped prior to attachment to the coldfinger/Dewar assembly. With the “shipped” SADA II coldfinger removed, the Dewar/sensor manufacturer inserts the now exposed expander cold end into the SADA II coldfinger which has been previously welded into the Dewar. The SADA II coldfinger which came attached to the cryocooler is shipped back to the cryocooler manufacturer for re-use.
While Stirling type expanders benefit from the fact they are unitary, the fact that their regenerator is a moving component is undesirable in that the movement can create unwanted system vibrations and potential mechanical failure points. It would therefore be desirable to have a unitary pulse tube expander with no moving parts that can act as a drop-in replacement for Stirling expanders in a SADA II coldfinger.